@swilli how are you testing speed? If it is from a download server it is probably limited by the server few servers allow unlimited downloads.
If it is on a speed test and the PC is connected by a direct ethernet cable to the hub (ie not via an extender, powerline adapter etc) AND nothing else is using bandwidth that is unusual but not unknown. Openreach's full fibre uses a system called GPON where up to 32 connections share a single fibre from the exchange which can carry 2.48Gb/s down which sounds horrendous but statistically is ample most of the time ss it is very rare for a customer to use the bandwidth they buy.. .
If the Sky connection test shows speeds below the guarantee speed of 400Mb/s for 3 days you can invoke tge guarantee see https://www.sky.com/help/articles/sky-broadband-speed-guarantee
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65inch Sky Glass, 3 Sky Streaming Pucks, Sky Ultrafast + and Sky SR213(white Wifi Max hub) main Wifi from 3 TP-Link Deco M4 units in access point mode
